swfshape::drawcurve() draws a quadratic curve
(using the current line style,set by swfshape::setline())
from the current pen position to the relative position
(anchorx,anchory)
using relative control point
(controlx,controly).
That is, head towards the control point, then smoothly turn to the
anchor point.

With 6 parameters, it draws a cubic bezier to point
(x+targetdx, x+targetdy)
with control points (x+controldx,
y+controldy) and
(x+anchordx, y+anchordy).